Exhaust driven turbochargers include a rotating group that includes a turbine wheel and a compressor wheel that are connected to one another by a shaft. The shaft is typically rotatably supported within a center housing by one or more bearings (e.g., oil lubricated, air bearings, ball bearings, magnetic bearings, etc.). During operation, exhaust from an internal combustion engine drives a turbocharger's turbine wheel, which, in turn, drives the compressor wheel to boost charge air to the internal combustion engine.
During operation, a turbocharger's rotating group must operate through a wide range of speeds. Depending on the size of the turbocharger, the maximum speed reached may be in excess of 200,000 rpm. Because of the wide operating range and the inherent design of the rotating group, most turbocharger rotating groups fit the definition of a “flexible rotor”. Flexible rotors require a unique balancing process to assure that residual unbalance in all balance planes are controlled and results verified with a test of the unbalance response throughout the operating range. A well balanced turbocharger rotating group is essential for proper rotordynamic performance. Efforts to achieve low levels of unbalance help to assure shaft stability and minimize rotor deflection which in turn acts to reduce bearing loads. Reduced bearing loads result in improved durability and reduced noise (e.g., as resulting from transmitted vibration).
To reduce vibration, turbocharger rotating group balancing includes component and assembly balancing. Individual components such as the compressor and turbine wheel assembly are typically balanced using a low rotational speed process while assembly (e.g., the completely assembled rotating group) are typically balanced using a high speed balancing process. Normally, the balance quality of the assembly is improved with a correction made on the compressor end of the rotating group alone.
Compressor wheel designs may be of two main types, those with a through bore and those without a through bore, which are referred to as “boreless”. For a compressor wheel with a through bore, the assembly process includes inserting a shaft in through the bore of the wheel and fixing the wheel to the shaft with a lock nut. The assembly is then installed in a high speed balancing machine for measurement and correction. The high speed balancer provides a means to operate the rotating group at the high speeds needed to provide adequate measurement and correction. Unbalance can be measured using instrumentation such as an accelerometer to provide an indication of unbalance in terms of vibration, or g's. In addition to the vibration response magnitude, the information provided by the high speed balancer can guide an operator, for example, by indicating where to remove material from the lock nut (e.g., phase angle of unbalance) to improve the balance. To measure unbalance phase, a high speed balancer may rely on a magnetic field sensor or an optical sensor. For a magnetic field sensor, the lock nut is magnetized (i.e., made of a magentizable material) whereas, for an optical sensor, one or more markings made on the lock nut or wheel may suffice. The magnetic method is generally preferred as being more accurate and reliable than the optical method.
For conventional boreless compressor wheels, unfortunately, the aforementioned magnetized lock nut approach to balancing does not apply. Boreless compressor wheels are often used for applications where high compressor wheel stresses make it beneficial to eliminate the bore through the wheel to reduce stress at the center of the wheel, which can be a source of failure at high rotational speeds. To balance a boreless compressor wheel, as other types of wheels, material must be removed. However, the only option for a boreless compressor wheel is to remove the material directly from the wheel itself. Accordingly, problems can arise when, after removal of some material, further balancing is required. For example, if during a final rotating group balancing operation, an acceptable balance cannot be achieved by further removal of material, the compressor wheel must be scrapped. Specifically, a nose of a boreless compressor wheel can often handle only a single balance cut and cannot be cut again.
Further, conventional boreless compressor wheels are typically made of aluminum, which is not a magentizable material. Accordingly, a magnetic field sensing approach to measuring unbalance cannot be used, which is unfortunate because, as mentioned, balancing approaches that use magnetization tend to be more efficient than optical approaches.
Various technologies described herein pertain to compressor wheels and nose pieces that can enhance balancing and, consequently, reduced rotating group vibration.